The present invention relates to a non-aqueous electrolyte secondary battery using a material capable of absorbing and releasing lithium as a negative electrode active material in which a lithium ion-conductive non-aqueous electrolyte is used, and in particular relates to a novel negative electrode active material and an electrolyte and a positive electrode active material suitable therefor for providing a novel secondary battery having a high voltage and a high energy density in which charge and discharge characteristics are excellent, the cycle life is long, and reliability is high.
The non-aqueous electrolyte battery which uses lithium as a negative electrode active material has been widely used in electric sources for a memory back-up, cameras and the like as a primary battery owing to advantages such as a high voltage, a high energy density, small self-discharge, excellent long-term reliability and the like. However, in recent years, in accordance with remarkable progress of compact type electronic instruments, communication instruments and the like, various instruments which require large current outputs for a battery as an electric source have been provided and from viewpoints of economics and realization of compact and light weight instruments, a secondary battery which is repeatedly rechargeable and has a high energy density is strongly demanded. Thus, active researches and developments have been done for developing a secondary battery from the above-mentioned non-aqueous electrolyte battery having the high energy density, a part of which is practically carried out, but still insufficient in the energy density, charge and discharge cycle life, reliability and the like.
In the prior art, as the positive electrode active material for constituting a positive electrode of such a secondary battery, there have been found the following three types of charge and discharge reaction. The first type includes metal chalcogenide such as TiS.sub.2, MoS.sub.2, NbSe, and the like, and metal oxide such as MnO.sub.2, MoO.sub.3, V.sub.2 O.sub.5, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, Li.sub.x Mn.sub.2 O.sub.4 and the like. In this type only lithium ion (cation) enters into and exits from spaces between layers and lattice positions or gaps between lattices of crystals in accordance with the intercalation and deintercalation reactions and the like. The second type includes conductive polymers such as polyaniline, polypyrrole, polyparaphenylene and the like, and in this type mainly only anion stably enters and exits in accordance with the doping and undoping reactions. The third type includes graphite intercalation compounds, conductive polymers such as polyacene and the like, and in this type both lithium cation and anion can enter and exit (in accordance with intercalation, deintercalation, or doping, undoping and the like).
On the other hand, as the negative electrode active material for constituting a negative electrode of such a battery, the electrode potential is basest when metal lithium is used alone, so that a battery combined with the positive electrode using the positive electrode active material as described above has the highest output voltage and also high energy density. However, a problem exists in that dendrite and passivation compounds are generated on the negative electrode in accordance with charge and discharge, deterioration due to charge and discharge is large, and the cycle life is short. In order to solve these problems, it has been proposed that materials capable of absorbing and releasing lithium ion be used as the negative electrode active material: (1) alloy of lithium and other metals such as Al, Zn, Sn, Pb, Bi, Cd and the like, (2) intercalation compounds or insertion compounds in which lithium ion is incorporated into a crystal structure of inorganic compounds such as WO.sub.2, MoO.sub.2, Fe.sub.2 O.sub.2, TiS.sub.2 and the like, graphite, carbonaceous materials obtained by calcining organic compounds and the like, and (3) conductive polymers such as polyacene, polyacetylene and the like in which lithium ion is doped.
Generally, however, when a battery is constituted by combining a negative electrode using a material capable of absorbing and releasing lithium ion other than metal lithium, as described above as the negative electrode active material, and a positive electrode using a positive electrode active material as described above, the electrode potential of such a negative electrode active material is nobler than that of metal lithium. The result is that there is such a drawback that an operating voltage of the battery is fairly lower as compared with a case in which metal lithium is used alone as the negative electrode active material. For example, the working voltage lowers by 0.2 to 0.8 V in the case of the use of an alloy of lithium and Al, Zn, Pb, Sn, Bi, Cd and the like, by 0 to 1 V in the case of a carbon-lithium intercalation compound, or by 0.5 to 1.5 V in the case of a lithium ion insertion compounds such as MoO.sub.2, WO.sub.2 and the like.
In addition, since elements other than lithium serve as negative electrode constituting elements, capacities per volume and per weight and the energy density considerably lowers.
Further, when the alloy of lithium and another metal of the above-mentioned (1) is used, there are such problems that the utilization efficiency of lithium is low during charge and discharge, and that the cycle life is short due to occurrence of cracks in the electrode to generate splits on account of repeated charge and discharge. In the case of the lithium intercalation compound or inclusion compound of (2), excessive charge and discharge cause deterioration such as decay of the crystal structure and generation of irreversible substances. In addition there is drawback that the output voltage of a battery using such materials is low because the electrode potential is high (noble) in many cases. In the case of the conductive polymer of (3), there is such a problem that the charge and discharge capacity, especially the charge and discharge capacity per volume, is small.
Thus, in order to obtain a secondary battery having a high voltage and a high energy density in which the charge and discharge characteristics are excellent and the cycle life is long, a negative electrode active material is necessary in which the electrode potential with respect to lithium is low (base), there is no deterioration such as decay of the crystal structure and generation of irreversible substances due to absorption and release of lithium ion during charge and discharge, and an amount capable of reversibly absorbing and releasing lithium ion that is, an effective charge and discharge capacity, is larger.
On the other hand, with respect to the above-mentioned positive electrode active materials, the first type generally has a large energy density but has a drawback that the deterioration is large due to decay of crystals, generation of irreversible substances and the like in case of excessive charge and excessive discharge. In addition, in the case of the second and the third types, on the contrary, there is such a drawback that the charge and discharge capacity, especially the charge and discharge capacity per volume and the energy density, are small.
Thus, in order to obtain a secondary battery having a high capacity and a high energy density in which the excessive charge characteristic and the excessive discharge characteristic are excellent, a positive electrode active material is necessary in which there is neither decay of crystals nor generation of irreversible substances due to excessive charge and excessive discharge, and the amount of lithium ion which is repeatedly absorbed and released is larger.